1. Field of the Invention
The present invention relates generally to control systems for vehicle engines coupled with automatic transmissions, and more particularly, to a system for controlling an output of a vehicle engine which is coupled with an automatic transmission employed in a vehicle so as to suppress torque shocks brought about by speed change operations performed in the automatic transmission.
2. Description of the Prior Art
There has been commonly used in vehicles an automatic transmission which is coupled with an engine provided in the vehicle and includes a torque converter comprising a pump impeller, a turbine runner and a stator, and a power transmitting gear arrangement including a speed change mechanism connected to the turbine runner in the torque converter. Such an automatic transmission employed to be coupled with the engine in the vehicle is usually accompanied with a hydraulic pressure control device including an oil hydraulic control circuit, by which engaging friction elements, such as clutch and brake elements, operating with oil pressure in the speed change mechanism are caused to work for a speed change operation.
When the speed change operation is carried out in the automatic transmission accompanied with the hydraulic pressure control device, sudden rise or down in speed of the engine is caused in response to variations in gear ratio in the speed change mechanism and thereby sudden variations in torque are caused on an output shaft of the automatic transmission. This results in that torque shock on the vehicle is brought about by each speed change operation performed in the automatic transmission.
With the intention of suppressing the torque shock resulting from the speed change operation, it is considered to adjust oil pressure applied to the engaging friction elements so that each engaging friction element comes into engagement or disengagement gradually and smoothly. In such a case, however, a period of time in which the engaging friction element is kept in a slipping condition in process of the speed change operation is protracted and therefore it is feared that the engaging friction element is subjected to seizing and abrasion of great degree.
In view of this, there has been proposed to reduce torque produced by the engine (engine torque) for a predetermined duration on the occasion of the speed change operation so as to suppress the torque shock resulting from the speed change operation, as disclosed in, for example, the Japanese patent application published before examination under publication number 61-104128. In the case where the engine torque is thus reduced for suppressing the torque shocks, for example, ignition timing is selected as a controlled subject for varying the engine torque and controlled to be retarded, compared with a normal ignition timing, so as to reduce the engine torque during the speed change operation.
Usually, in the vehicle which is equipped with the engine coupled with the automatic transmission, a variation in gear ratio in the speed change mechanism caused on the occasion of a speed change between high speed ranges each provided with a relatively small gear ratio is smaller than a variation in gear ratio in the speed change mechanism caused on the occasion of a speed change between low speed ranges each provided with a relatively large gear ratio. Further, in a condition in which the high speed range is taken in the speed change mechanism, rotary inertia on an output shaft of the automatic transmission is relatively large compared with that in a condition in which the low speed range is taken in the speed change mechanism, so that variations in torque on the output shaft of the automatic transmission are restricted and therefore the torque shock on the vehicle is substantially reduced.
Besides, when the high speed range is taken in the speed change mechanism, the vehicle is, in general, traveling at high speed and therefore it is to be understood that to prevent the engine torque from being reduced takes precedence of to suppress the torque shock resulting from the speed change operation. Especially, in the case where a shifting-down operation for causing a change from a speed range of the smallest gear ratio, for example, the fourth speed range to a speed range of the second smallest gear ratio, for example, the third speed change is carried out in the automatic transmission, the vehicle is likely to require to be accelerated to pass another vehicle ahead and therefore it is desirable that the control for reducing the engine torque so as to suppress the torque shock resulting from the speed change operation is not performed.
However, in the prior art, the above mentioned differences between the speed change operation causing the change from one to another of the low speed ranges and the speed change operation causing the change from one to another of the high speed ranges, has not been taken into consideration and a controlled variable in the control for reducing the engine torque so as to suppress the torque shock resulting from the speed change operation is set to be constant regardless of a mode of the speed change operation. Accordingly, in the case where the speed change between the high speed ranges is brought about in the automatic transmission, it is feared that the controlled variable in the control for reducing the engine torque so as to suppress the torque shock resulting from the speed change operation is large excessively and thereby excessive reduction of the engine torque is induced so that required engine torque can not be obtained.